Cell fusion between mitotic and interphase cells leads to premature chromosome condensation of the interphase nucleus under the influence of certain factors present in the mitotic cells. Our preliminary studies indicate that these factors are Ca++-sensitive, Mg++-dependent and heat labile proteins. These mitotic proteins, when injected into immature Xenopus laevis oocytes, induce germinal vesicle (nucleus) breakdown and chromosome condensation. In this application we propose to probe the nature of these mitosis-specific proteins by taking an immunological approach. In addition, we also would like to study the role of H1 histone phosphorylation in chromosome condensation. Hence, the specific aims of this proposal are: (1)\to produce monoclonal antibodies that react specfically with mitotic HeLa cells; and (2)\to provide direct evidence for the role of H1 histone phosphorylation in chromosome condensation by using the massive cell fusion technique that we developed recently. Lately, we have been successful in isolating two hybridoma clones that produce antibodies that react specifically with mitotic and meiotic cells from every species tested. Chromosomes as well as cytoplasm in mitotic cells reacted with the antibodies, as detected by indirect immunofluorescence. The two antibodies, designated MPM-1 and MPM-2, recognize a family of polypeptides with apparent molecular masses of 40 to greater than 200 kilodaltons (KD). Both antibodies reacted strongly with three polypeptide bands of 70, 118 and 182 KD on polyacrylamide slab gels transferred to nitrocellulose sheets. These bands were found to be phosphoproteins as shown by 32P labeling and autoradiography and their removal by alkaline phosphatase treatment. Studies dealing with the microinjection of these antibodies into mammalian cells and the role of histone H1 phosphorylation in premature chromosome condensation are in progress.